Plasmids of incompatibility group P (IncP) show an extraordinary host range among gram-negative bacteria. The genetic and molecular basis for this potential is not known, and we are studying in detail the IncP plasmid RK2 in the belief that it may reveal special genetic functions or unique strategies for gene expression. We have found that RK2 encodes two novel sets of genetic determinants: (1) kil genes (kilA, kilB, kilC) which are lethal to an E, coli host, and (2) kor genes (korB, korC) which control the kil genes. Our studies indicate that these may be encoded by all IncP plasmids, and we have speculated that these may be host range determinants. We have learned that these genes are involved in an array of regulatory interactions, including the control of plasmid regulation. Here we wish to continue our investigations of these genes. Specifically we intend to do the following: (1) to complete the genetic mapping and to determine the nucleotide sequences of each of the kil and kor determinants, (2) to identify and characterize the gene products, (3) to learn how kil functions are lethal to E coli, (4) to determine the molecular mechanisms by which the kor functions prevent cell death by the kil genes, (5) to determine the molecular basis of korA-mediated activation of korB and korC expression, (6) to study the evolutionary relationships of IncP plasmids by hybridization analysis and nucleotide sequence comparisons of their kil and kor determinants, and (7) to test the relationship of host range to kil and kor functions. By employing a variety of both genetic and molecular techniques, we hope to achieve a complete understanding of the kil and kor functions and their importance to the biology of IncP plasmids.